Thymine starvation of RecA+ Thy- strains of Salmonella typhimurium does not induce the inhibition of cellular respiration, one of the recA-gene dependent SOS functions. Nevertheless, thymine deprivation is able to produce a normal induction of prophage and thymineless death in these same strains. However, when these mutants are treated, in the presence of thymine, with UV-irradiation or bleomycin, they show a normal inhibition of cellular respiration and other SOS functions. Thus, one injurious treatment (thymine deprivation) may trigger prophage induction but not cessation of respiration, whereas another agent (UV-irradiation) may induce both. Together, these results suggest a possible discrimination in the pathways and conditions of expression of various SOS functions. © 1982 H. Veenman & Zonen.